1. Field of the Invention
The present invention relates to optical transmission systems, more particularly to a multi-point optical transmission system for optically transmitting code-division multiplex signals in an analog fashion from a plurality of slave stations to a master station through an optical fiber.
2. Description of the Background Art
FIG. 4 is a block diagram showing an exemplary functional configuration of a conventional multi-point optical transmission system.
The system in FIG. 4 includes radio base stations 401 to 40n (where n is an arbitrary integer of 2 or more) and a switching station 41, and each of the radio base stations 401 to 40n is connected to the switching station 41 via an optical fiber 43. Each radio base station 401 to 40n includes an antenna 401, a driving section 402, and an uplink electrical-optical converting section 403. The switching station 41 includes uplink optical-electrical converting sections 4111 to 411n, demodulating sections 4121 to 412n, and a switching section 413.
The antenna 401 receives an uplink radio signal. The uplink radio signal herein is a code-division multiplex signal into which radio signals outputted from a plurality of terminals (not shown) in each cell 42 are multiplexed in a code-division system. Each uplink radio signal is equal in frequency as predetermined. The driving section 402 applies a bias to the uplink radio signal. The uplink electrical-optical converting section 403 converts the uplink radio signal into an optical signal whose intensity is modulated by the uplink radio signal. The uplink optical-electrical converting sections 4111 to 411n convert the optical signal into an electrical signal (an uplink radio signal). The demodulating sections 4121 to 412n demodulate the uplink radio signal outputted from the radio base stations 401 to 40n to base band digital data (Note that a process of demodulation herein includes xe2x80x9creverse diffusionsxe2x80x9d. Specifically, a radio signal is subjected to reverse diffusion before demodulation so as to obtain base band digital data; the same is applicable to the description below). The switching section 413 goes through a switching process in accordance with the base band digital data.
The operation whereby the system in FIG. 4 optically transmits a plurality of uplink radio signals outputted from the radio base stations 401 to 40n to the switching station 41 in a multi-point fashion is described next below. Each cell 42 in the system in FIG. 4 includes a plurality of terminals (not shown), and each of the terminals transmits a radio signal in a code-division multiplex system to one of the radio base stations 401 to 40n located in the same cell 42. Thereafter, each of the uplink radio signals obtained after the code-division multiplexing of radio signals from the terminals is then received by the antenna 401 of the respective radio base stations 401 to 40n.
The received uplink radio signals are respectively biased in the driving section 402, and sent to the uplink electrical-optical converting section 403. In response thereto, the uplink electrical-optical converting section 403 outputs an optical signal whose intensity is modulated by the uplink radio signal. In this manner, each optical signal outputted from the radio base stations 401 to 40n is transmitted to the switching station 41 through the optical fiber 43. Each of the transmitted optical signals is subjected to optical-electrical conversion in the uplink optical-electrical converting sections 4111 to 411n. Each electrical signal obtained after the conversion (uplink electrical signals outputted from the radio base stations 401 to 40n) is demodulated to base band digital data in the demodulating sections 4121 to 412n, and then sent to the switching section 413. The switching section 413 goes through a switching process in accordance with the respective base band digital data.
As will be known from the above, a plurality of uplink radio signals outputted from the radio base stations 401 to 40n can be optically transmitted in a multi-point fashion to the switching station 41 in the system in FIG. 4.
The system, however, necessitates the uplink optical-electrical converting sections 4111 to 411n as many as the radio base stations 401 to 40n in the switching station 41. Consequently, if the system has a large number of cells 42, the switching station 41 accordingly becomes larger and costs more.
Thus, another type of multi-point optical transmission system was proposed, in which, with only a single optical-electrical converting section provided in a switching station, uplink radio signals outputted from a plurality of radio base stations are optically transmitted to the switching station in a multi-point fashion.
FIG. 5 is a block diagram showing an exemplary configuration of another conventional multi-point optical transmission system.
The system in FIG. 5 includes radio base stations 501 to 50n (where n is an arbitrary integer of 2 or more) and a switching station 51, and each of the radio base stations 501 to 50n is connected to the switching station 51 via an optical fiber 53. Each of the radio base stations 501 to 50n includes an antenna 501, a driving section 502, and an uplink electrical-optical converting section 503. The switching station 51 includes an optical multiplexing section 511, uplink optical-electrical converting section 512, a branching section 513, demodulating sections 5141 to 514n, and a switching section 515.
The optical multiplexing section 511 multiplexes optical signals outputted from the radio base stations 501 to 50n. The uplink optical-electrical converting section 512 converts an optical signal obtained by multiplexing into an electrical signal. The branching section 513 branches the electrical signal into n signals. Other components in this system perform each task in a similar manner to those in the system in FIG. 4.
The operation whereby the system in FIG. 5 optically transmits a plurality of uplink radio signals outputted from the radio base stations 501 to 50n in a multi-point fashion to the switching station 51 is described next below.
Referring to FIG. 5, each cell 52 in the system includes a plurality of terminals (not shown), and each of the terminals transmits a radio signal in a code-division multiplex system to one of the radio base stations 501 to 50n located in the same cell 52. Each of the radio signals obtained after the code-division multiplexing is then received by the antenna 501 of the respective radio base stations 501 to 50n. The received uplink radio signals are respectively biased in the driving section 502, and then sent to the uplink electrical-optical converting section 503. In response thereto, the uplink electrical-optical converting section 503 outputs an optical signal whose intensity is modulated by the uplink radio signal.
In this manner, each of the optical signals outputted from the base radio stations 501 to 50n is transmitted to the switching station 51 through the optical fiber 53. The transmitted optical signals are multiplexed in the optical multiplexing section 511, and a signal obtained by multiplexing is then subjected to optical-electrical conversion in the uplink optical-electrical converting section 512. An electrical signal obtained by the conversion is branched into n signals in the branching section 513. The respective electrical signals obtained by n-branching are inputted to the demodulating sections 5141 to 514n, and the uplink radio signals outputted from the radio base stations 501 to 50n are selectively demodulated to base band digital data therein. The base band digital data is then respectively sent to the switching section 515, and a switching process is performed therein in accordance with the respective base digital data.
As will be known from the above, unlike the system in FIG. 4, the system in FIG. 5 having only a single uplink optical-electrical converting section 512 in the switching station 51 can optically transmit a plurality of uplink radio signals outputted from the radio base stations 501 to 50n in a multi-point fashion to the switching station 51.
The system, however, causes a problem when demodulating the respective electrical signals obtained by n-branching to base band digital data in the demodulating sections 5141 to 514n. That is, as the electrical signals obtained by n-branching each include a plurality of uplink radio signals outputted from the radio base stations 501 to 50n, the uplink radio signals disturb, for example, a signal obtained after reverse diffusion (corresponds to an uplink radio signal outputted from the radio base station 501) as a noise when demodulation is taken place in the demodulating section 5141, thereby causing a drop in a C/N ratio.
For reference purposes, FIGS. 6a and 6b show two spectrums of a signal before and after the reverse diffusion taken place in the demodulating section 5141. FIG. 6a shows a spectrum before reverse diffusion, and FIG. 6b shows a spectrum after reverse diffusion. As shown in FIG. 6, an uplink radio signal 61 is subjected to reverse diffusion in the demodulating section 5141 but not the other two uplink radio signals 62 and 63. In this case, non-reverse-diffused uplink radio signals 62 and 63 disturb a signal 61A obtained by the reverse diffusion as a noise.
Therefore, an objective of the present invention is to provide a multi-point optical transmission system, in which, with only a single uplink optical-electrical converting section provided in a master station, code-division multiplex signals outputted from a plurality of slave stations are optically transmitted to the master station in a multi-point manner without a drop in each C/N ratio thereof.
The present invention has the following features to attain the objective above.
A first aspect of the present invention is directed to a multi-point optical transmission system for optically transmitting code-division multiplex signals from a plurality of slave stations to a master station,
the slave stations having each varied carrier frequency assigned, each of the slave stations comprising:
frequency conversion means for subjecting each code-division multiplex signal to be transmitted to the master station to frequency conversion to equalize the predetermined frequency thereof with the carrier frequency assigned to one own station;
drive means for applying a bias to a signal obtained after the conversion by the frequency conversion means; and
electrical-optical conversion means for converting an electrical signal obtained through the bias applied by the drive means into an optical signal whose intensity is modulated by the electrical signal, and
the master station comprising:
optical multiplex means for multiplexing optical signals obtained after conversion by the electrical-optical conversion means;
optical-electrical conversion means for converting an optical signal obtained after multiplexing by the optical multiplex means into an electrical signal;
band pass filter means for extracting a signal equal in frequency to the carrier frequency assigned to each of the slave stations from the electrical signal obtained after conversion by the optical-electrical conversion means; and
frequency re-conversion means for subjecting each signal extracted by the band pass filter means to frequency re-conversion to equalize each frequency thereof with a predetermined frequency.
As described above, in the first aspect of the present invention, each signal to be demodulated by the demodulation means includes only a code-division multiplex signal outputted from a single slave station. Accordingly, a code-division multiplex signal outputted from a desired base station will not be disturbed by code-division multiplex signals outputted from the other stations as a noise any more. Therefore, with only a single optical-electrical conversion means provided in the master station, the multi-point optical transmission system of the present invention can successfully optically transmit code-division multiplex signals outputted from a plurality of slave stations to a master station in a multi-point fashion without causing a drop in each C/N ratio thereof.
A second aspect of the present invention is directed to the multi-point optical transmission system as set forth in the first aspect of the invention, wherein each of the slave stations has varied downlink carrier frequency further assigned,
the master station further comprises:
downlink frequency conversion means for subjecting each downlink signal to be transmitted to the slave stations to frequency conversion to equalize the other predetermined frequency thereof with the downlink carrier frequency assigned to each of the slave station;
multiplex means for multiplexing downlink signals obtained after the conversion by the downlink frequency conversion means;
downlink drive means for applying a bias to a signal obtained after multiplexing by the multiplex means;
downlink electrical-optical conversion means for converting an electrical signal obtained through the bias applied by the downlink drive means into an optical signal whose intensity is modulated by the signal; and
optical branch means for branching the optical signal obtained after conversion by the downlink electrical-optical conversion means, and
each of the slave stations further comprises:
downlink optical-electrical conversion means for converting one of a plurality of optical signals obtained after branching by the optical branch means into an electrical signal;
downlink band pass filter means for extracting a signal equal in frequency to a downlink carrier frequency assigned to the own station from the electrical signal obtained after conversion by the downlink optical-electrical conversion means; and
downlink frequency re-conversion means for subjecting the downlink signal extracted by the downlink band pass filter means to frequency re-conversion to equalize a frequency thereof with a frequency before the frequency conversion.
As described above, in the second aspect of the present invention, each downlink signal outputted from the master station can further be optically transmitted to the slave stations in a multi-point fashion without a drop in each C/N ratio thereof.
A third aspect of the present invention is directed to the multi-point optical transmission system as set forth in the second aspect of the invention, wherein each of the slave stations has varied pilot signal frequency further assigned, and
further comprises pilot signal multiplex means for multiplexing a pilot signal having a pilot signal frequency assigned to the own station on a code-division multiplex signal to be transmitted to the master station,
the master station further comprises downlink pilot signal multiplex means for multiplexing a downlink pilot signal on each downlink signal to be transmitted to the slave stations,
the downlink pilot signal multiplex means examines the electrical signal obtained after conversion by the optical-electrical conversion means to measure power of the pilot signal, and then generates control information indicating a difference between a measured value and a predetermined threshold to apply the same as a modulation component to the downlink pilot signal to be multiplexed,
the downlink band pass filter means further extracts the downlink pilot signal from the electrical signal obtained after the conversion by the downlink optical-electrical conversion means, and
in accordance with the control information supplied to the downlink pilot signal extracted by the downlink band pass filter means, the drive means adjusts power of a bias to be applied so that power of the code-division multiplex signal at time of reaching the master station are equalized.
As described above, in the third aspect of the present invention, an amount of noise and distortion occurs in an optical transmission system is reduced by multiplexing a pilot signal into each code-division multiplex signal and a downlink pilot signal into each downlink signal. Moreover, the system of the present invention suppresses noise-to-noise (power thereof) variation occurred in each code-division multiplex signal by equalizing each power thereof at the time of reaching the master station. As a result, the transmission characteristic of the system is improved.
A fourth aspect of the present invention is directed to the multi-point optical transmission system as set forth in the third aspect of the invention, wherein when converting the electrical signal into the optical signal, the electrical-optical conversion means adjusts each optical modulation level of the optical signal to equalize the same.
As described above, in the fourth aspect of the present invention, the equalized power of the optical signals to be transmitted to the optical-electrical conversion means prevents a noise having a relative intensity from occurring only to a particular code-division multiplex signal. As a result, the transmission characteristic of the system is improved.
A fifth aspect of the present invention is directed to the multi-point optical transmission system as set forth in the third aspect of the invention, wherein each of the code-division multiplex signals is a signal into which a plurality of radio signals transmitted from a mobile station in each cell of the slave stations are code-division-multiplexed,
each downlink signal is a signal into which a plurality of radio signals to be transmitted to the mobile station in the cell are multiplexed, and
each of the slave stations further comprises:
an antenna for transmitting and receiving the radio signals to and from the mobile station located in one own cell; and
a circulator for supplying an output from the antenna to the frequency conversion means and an output from the downlink frequency re-conversion means to the antenna.
As described above, in the fifth aspect of the present invention, the slave stations can respectively be downsized as the system does not require thereto to have two antennas for transmitting and receiving signals, respectively.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.